1. Field of the Invention
This invention relates to an apparatus for driving a piezoelectric actuator in a fuel injection valve or other devices.
2. Description of the Prior Art
Some fuel injection valves for internal combustion engines include valve members driven by actuators having piezoelectric elements. In such a piezoelectric actuator, a high voltage is selectively applied to a piezoelectric element. Generally, a portion of a circuit for generating a high voltage includes a combination of a dc power source and a switching element which provides dc current pulses fed to a capacitor via an inductor to charge the capacitor.
Electric parameters, especially a capacitance, tend to vary from piezoelectric element to piezoelectric element. In addition, the electric parameters tend to vary in accordance with an ambient temperature. Accordingly, it is desirable to choose various constants of a drive circuit for a piezoelectric element in consideration of such variations in the electric parameters thereof. In cases where cylinders of an internal combustion engine are provided with respective fuel injection valves of the piezoelectric type, it is desirable to compensate for variations in the electric parameters from piezoelectric element to piezoelectric element.
Japanese published unexamined patent application 62-17338 discloses a drive circuit for electrostrictive actuators which control fuel injection valves respectively. In the drive circuit of Japanese patent application 62-17338, a switching element and inductors are used in common to the electrostrictive actuators to compensate for variations in response characteristics from actuator to actuator.
Specifically, in the drive circuit of Japanese patent application 62-17338, a first common switching element and a first common inductor are connected in series with a capacitor charged by a dc power source. The first common switching element is periodically activated so that the first common inductor derives a high voltage from a voltage across the capacitor. The high voltage is applied in common to the electrostrictive actuators to charge them. Second switching elements are connected to the electrostrictive actuators respectively while a second inductor is connected in common to the electrostrictive actuators. In more detail, opposite ends of each of the electrostrictive actuators are connected via a second switching element and the second common inductor. The second switching elements are sequentially activated so that the electrostrictive actuators will be sequentially discharged via the second common inductor.
Japanese published unexamined patent application 62-75035 discloses a drive circuit for a piezoelectric actuator which controls an engine fuel injection pump. In the drive circuit of Japanese patent application 62-75035, a voltage generated by the piezoelectric element in response to a fuel-pressure stress is used in charging a capacitor during a first period, and the voltage across the capacitor is used in charging the piezoelectric element during a second period. This process saves electric energy.